Vibration isolation has long been used in mechanical systems to reduce vibration of elements which are sensitive to severe vibration. Flexible supporting devices called vibration isolators, typically employing elastomeric springs, are commonly used to support equipment items. Their purpose is to reduce the transmission of vibration between an equipment item and its supporting structure. The vibration source may be within the equipment, such as a motor, and the isolation serves to reduce the vibration transmitted to the supporting structure. Or, the equipment may need protection from vibration emanating from the supporting structure. In some cases viscoelastic spacers, such as rubber washers, have been disposed in the transmission path between a source of vibration and a coupled object to reduce the transmitted vibrations to the object. The rubber washers favorably change the resonant vibration characteristics of the system and absorb energy to effect vibration reduction where desired. For example, U.S. Pat. No. 5,350,173 discloses a resilient insert installed into the rim of a tennis racket head to reduce head vibrations during play.
In a submarine application, the hull supports an equipment truss containing sources of vibration. The truss is made of tubular members which have been experimentally filled with polyethylene beads for reducing noise vibration transmitted to the hull and generated in to prevent underwater sonar detection of the submarine. Low-density polyethylene beads were used for internal vibration energy absorption to reduce the propagation of vibration noise over a broadband of high frequencies. Unlike the invention herein disclosed, the particulate beads were not suspended within a tuned mass to create a tuned vibration absorber to be maximally effective in a desired frequency band.
In an application to a DC-9 aircraft, four suspended all-metal tuned vibration absorbers were elastically attached to the fuselage at the forward attachment points of the engines. The purpose of the absorbers was to reduce fuselage vibration resulting from vibration induced by unbalances of rotating shafts within the engines. The desired result, to significantly reduce the noise disturbing passengers at the rear of the cabin was achieved. The all-metal absorbers had limited energy absorption capacity which led to a narrower than desired frequency bandwidth of vibration reduction. Each suspended absorber mass was threaded onto a metal suspension rod which served as the spring. The variable positioning of the mass along the rod allowed fine tuning of the frequency of maximum absorption. The disadvantage resulting from the limited damping of these absorbers was the greater than desired loss of effectiveness of the absorbers due to slight mistuning. This imposed an undesirable tight tolerance on initial tuning and limited the length of service between adjustments of tuning to maintain effectiveness. For these reasons the absorbers saw limited application by the airlines.
In a rocket application, high vibrations are experienced during transonic flight due to high fluctuating aerodynamic pressure on the rocket skin. In a compartment of the vehicle containing equipment vulnerable to the induced vibration, the skin is supported by axially oriented stringers which are I-beams and serve to support the skin. A plurality of such stringer I-beams are equiangularly attached and disposed in parallel to support the interior of the rocket skin and also serve to support internal trusses which carry sensitive electronic payload equipment items. At vibration resonances of the skin-stringer-truss-equipment mechanical system, the vibration response of the equipment tends to be most intense and can cause mechanical damage or improper equipment operation, for example navigation errors of a guidance computer. Viscoelastic damping material was applied to skin, stringers, and truss to dissipate vibrational energy but failed to provide sufficient vibration reduction in an especially critical band of frequencies. This lack of effectiveness and other disadvantages were corrected using the present invention.